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Which airliners are able to use APU while airborne?

Posted: Sat Oct 02, 2021 9:48 pm
by convair880mfan
According to author Jim Upton in his book, "Lockheed L-1011 Tristar" :

"Providing ground self-sufficiency and emergency in-flight power, the (L-1011) APU system is operable up to an altitude of 30,000 feet under all flight conditions within the aircraft operation envelope." (1)

Are there other aircraft that can use the APU while airborne or can they all? Thanks.

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(1) Jim Upton. Lockheed L-1011 Tristar, in Airliner Tech Series Vol. 8. North Branch, MN: Specialty Press, 2001. p. 28.

Re: Which airliners are able to use APU while airborne?

Posted: Sat Oct 02, 2021 10:19 pm
by GalaxyFlyer
Just about all of the twins can, the B727 and B747 couldn’t.

Re: Which airliners are able to use APU while airborne?

Posted: Sat Oct 02, 2021 11:11 pm
by convair880mfan
Did the Lockheed C5 Galaxy have an APU and could it be used in flight, GalaxyFlyer?

Re: Which airliners are able to use APU while airborne?

Posted: Sat Oct 02, 2021 11:42 pm
by GalaxyFlyer
Two APUs, L and R. Useable in-flight but could cause paint damage from the exhaust on A models, larger shield on B, IIRC. Little need to run one in flight, 10 hydraulic pumps, two off bleed air, 2 per engine. Four generators plus hydraulic emergency generator.

Re: Which airliners are able to use APU while airborne?

Posted: Sat Oct 02, 2021 11:57 pm
by BoeingGuy
GalaxyFlyer wrote:
Just about all of the twins can, the B727 and B747 couldn’t.


I thought the 747 APU could be used in air; it just couldn’t be started in air.

Re: Which airliners are able to use APU while airborne?

Posted: Sun Oct 03, 2021 12:44 am
by GalaxyFlyer
That maybe true, but not starting airborne is a pretty severe handicap.

Re: Which airliners are able to use APU while airborne?

Posted: Sun Oct 03, 2021 5:32 pm
by Flow2706
On the A320 APU can be started and run up to the service ceiling (39800ft or 39100ft, except for some ACJs). The generator can be used at altitude as well, it has the same rating as the engine generators. APU bleed usage is limited to around 20000 or 22000ft (depending on APU type) and APU bleed may not be used for wing anti ice (as it is too cold).

Re: Which airliners are able to use APU while airborne?

Posted: Mon Oct 04, 2021 2:34 pm
by gregorygoodwin
Does the APU have to be able to start and run during flight have anything to do with being category 3 flight able?

Re: Which airliners are able to use APU while airborne?

Posted: Mon Oct 04, 2021 3:39 pm
by Woodreau
You need two independent electrical sources. So an inop APU doesn’t preclude Cat III operations.

If you only have a single electrical source, i.e. IDG 2 disconnected with an inop APU the equipment review for Cat III operations allows Cat III Single (fail passive) approach but not a Cat III Dual (fail operational) approach.


Edit: for an A320 series aircraft I don’t know how other aircraft are impacted.

Re: Which airliners are able to use APU while airborne?

Posted: Tue Oct 05, 2021 8:37 am
by Max Q
MD80, 757 and 767 APU all run and can be started in flight. The latter two can be used for electrical needs up to the aircraft ceiling




If dispatching with an engine driven generator inop we would power that side with the APU generator and it would be left running for the entire flight


We’d occasionally get requests from maintenance control to start the APU halfway through a long haul flight. Even after five hours cold soaking at high altitude they always started up immediately


On an autoland approach we would start the APU as a back up in case one aircraft generator failed

Re: Which airliners are able to use APU while airborne?

Posted: Tue Oct 05, 2021 9:10 am
by bigb
BoeingGuy wrote:
GalaxyFlyer wrote:
Just about all of the twins can, the B727 and B747 couldn’t.


I thought the 747 APU could be used in air; it just couldn’t be started in air.


This is correct. It is only available for bleed air up to 15,000. Mainly for Pack to APU takeoffs. After 15,000 it is no longer available for air but can stay on until 20k. However, it can not be started in the air. This is for the 747.

Apparently, APU starting inflight on the 747 wasn’t certified, hence the limitation.